FIG. 1 is a block diagram of a conventional system 10 for printing halftoned images. In such a system 10 a storage device 12 provides continuous tone (contone) images to a spooler 14. The spooler converts the image to a halftoned image which is typically scaled by scaler 22. The scaled halftoned image is then provided to a control unit 18 of the printer 16. A processor 20 processes the scaled halftoned image and prints it on a document.
Scaling halftoned images is required in cases where the resolution of the original image does not match the desired resolution of the output medium. This problem exists, for example, in some conventional printers, where the print heads are configured for 604 dpi, but customers have large databases of images in 600 dpi. Periodic pixel repetition solves this problem acceptably for text and line art, but produces low-frequency (and thus, highly visible) artifacts for prescreened images, degrading image quality significantly.
In one inkjet printer, the halftones have already been pre-calculated. This calculation is time-intensive, and thus, recalculation cannot be done in real time. Additionally, pre-screened images may use either stochastic, dispersed, or clustered-dot halftones, depending on the database of the customer. Furthermore, there will be no printer system control over the structure of these pre-screened halftones.
In “Non-Integer Scaling of Raster Images”, U.S. Pat. No. 6,226,420 (Filed: Dec. 26, 1997 Hamil), 240 dpi to 600 dpi scaling is achieved with a special pel repetition pattern that produces high-quality image. However, the 600 dpi to 604 dpi scaling required by a conventional printer would require (a) a completely new pel repetition pattern (b) that is very large in order to avoid low-frequency artifacts. Even the above-identified scaling results in low-frequency artifacts in some pre-screened data.
This problem has also been addressed by adding an extra line of pels every 0.25 inch. This method works well for large text, acceptably but not terrifically for small or fine text, and very poorly for halftoned images.
The traditional way of addressing such a problem involves descreening, scaling, and rescreening. An example of this method is described in the paper entitled, “New Results on Reconstruction of Continuous-Tone from Halftone,” by Anaboui et al in IEEE paper 0-7803-0532-9/92 (1992). However, this method results in reduction of image resolution (if the descreening filter is too course), or in the potential introduction of Moiré patterns (if the descreening filter is too fine). Many applications will be unlikely to tolerate these kinds of artifacts in their images.
Accordingly, what is needed is a system and method which provides a high quality halftoned image with minimal artifacts. The system and method should be compatible with image printing architectures, cost effective and easily implementable. The present invention addresses such a need.